1. Field of the Invention
The present invention relates to a magnetic sensor having a GMR magnetic laminated film, more particularly, a magnetic sensor with an improved thermal stability.
2. Description of the Related Art
In a magneto resistive element (hereinafter referred to as an MR element) used in a magnetic sensor in a magnetic encoder, a NiFe alloy film or NiCo alloy film utilizing an anisotropic magneto resistive effect is used. Such materials have a magnet resistance ratio due to the anisotropic magneto resistive effect on the order of 2.5%. An output (signal voltage) of the magnetic encoder using the MR element ranges from 40 to 50 mV. Increasing a magnetic gap of the magnetic encoder used in a machine tool reduces the output. In order to obtain a sufficient output of the magnetic sensor even if the increased magnetic gap, the magneto resistance ratio (xcex94R/R) is required to become larger.
Recently, there has been contemplated that a magnetic laminated film of [Nixe2x80x94Fexe2x80x94Co/Cu] or [Nixe2x80x94Fe/Cu], each of which is a giant magneto resistive element (GMR element) having a large magneto resistance ratio, is used as the magnetic sensor. For example, the Japanese Patent No. 2812042 discloses a magnetic sensor of a metal superlattice film having a NiCoFe layer and a nonmagnetic metal thin layer laminated with each other. According to the disclosure, in this magnetic sensor, directions of a current and an applied magnetic field are parallel with each other, and thus the magneto resistance ratio can be increased to 15 to 20%. Such a GMR element typically has the magneto resistance ratio two to four times larger than that of the MR element made of a Nixe2x80x94Fe alloy or Nixe2x80x94Co alloy film using the anisotropic magneto resistive effect. Since the output signal of the magnetic sensor can be increased when the magneto resistance ratio is increased, using the GMR element enables the output two or more times higher than that of the MR element (80 to 100 mV, or higher) to be obtained. With the higher output, mounting can be accomplished with a widened magnetic gap when the magnetic encoder is assembled. With the widened magnetic gap, ease of assembly is enhanced, and therefore the production yield is enhanced. Besides, through the application of the GMR element with the higher output, a fill bridge circuit that currently involves the MR element can be replaced with a half bridge circuit that involves the GMR element, thereby miniaturizing the magnetic sensor.
However, in a manufacturing process of the magnetic sensor for the magnetic encoder, the magnetic sensor may be heated in the step of soldering wires, the step of forming a protective film, or the like after the magnetic laminated film (GMR element) is fabricated. In the soldering step, for example, heat used for applying solder to a terminal section of the magnetic sensor or soldering a flexible wiring board to the terminal section may be conducted into the magnetic sensor. Thus, the magnetic laminated film having already been fabricated may be heated.
The conventional magnetic laminated film (GMR element) of [Nixe2x80x94Fexe2x80x94Co/Cu] or [Nixe2x80x94Fe/Cu] has an insufficient thermal resistance compared to the MR element, and if the magnetic sensor is heated in the steps after the magnetic laminated film is formed, the magneto resistance ratio of the magnetic laminated film is decreased. It is considered that lead-free solder (having a composition free of Pb) will be used as the solder applied to a terminal of the magnetic sensor in the future. Compared to an eutectic point of lead solder that is 183 degrees centigrade in the case of Snxe2x80x94Pb solder, the eutectic point of the lead-free solder is higher, specifically, 221 degrees centigrade in the case of Snxe2x80x94Ag eutectic solder. As for the conventional magnetic laminated film of [Nixe2x80x94Fexe2x80x94Co/Cu] or [Nixe2x80x94Fe/Cu], xcex94R/R is decreased by about 10% when it is heated at the temperature of 230 degrees centigrade, so that the lead-free solder is difficult to use.
The GMR magnetic laminated film is used in the magnetic encoder. The magnetic encoder is used for a machine tool, precision machine, and optical instrument. Particularly in the precision machine and optical instrument, miniaturization of the magnetic sensor is required. For the miniaturization, a glass substrate is reduced in thickness compared to conventional one. In addition, an aluminum oxide (alumina) film or silicon oxide film is formed on a part of the protective film by sputtering. When the glass substrate is used and the protective film is formed by sputtering, the temperature of the substrate is raised to 180 to 190 degrees centigrade. If the glass substrate is reduced in thickness for the sake of miniaturization, it is difficult for heat to escape from the substrate to the sputtering apparatus, so that the temperature of the substrate is considered to be raised to 200 degrees centigrade or more. In the case of the magnetic laminated film of [Nixe2x80x94Fexe2x80x94Co/Cu] or [Nixe2x80x94Fe/Cu], if it is heated to a temperature of 200 degrees centigrade or more, the magneto resistance ratio thereof is decreased. For example, if it is heated at 250 degrees centigrade for one hour, the magneto resistance ratio (xcex94R/R) is decreased by as much as 20%. If such a magnetic laminated film undergoes the manufacturing process of the magnetic encoder, the magnetic encoder with a high output can be hardly provided.
Accordingly, an object of the present invention is to provide a magneto resistive sensor having a magnetic laminated film with a large magneto resistance ratio and improved thermal resistance and a magnetic encoder using the same.
The magneto resistive sensor according to the invention includes a nonmagnetic substrate, an underlayer deposited on the substrate, and a magnetic laminated film formed on the underlayer, in which the magnetic laminated film has a plurality of magnetic thin layers and a plurality of nonmagnetic thin layers alternately laminated. The magnetic thin layer is 5 to 30 angstrom thick, and the nonmagnetic thin layer is 5 to 30 angstrom thick. The magnetic thin layer has a composition represented by the formula: [(NixCo1-x)yFe1-y]zB1-z, where 0.50 less than xxe2x89xa61.00, 0.70xe2x89xa6y less than 1.00, 0.90xe2x89xa6z less than 1.00. Preferably, in the formula of composition, x follows the inequality: 0.60 less than x less than 1.00.
The formula of composition of the magnetic thin layer is represented in terms of atomic contents ratio. The atomic ratio of Ni content to the sum of Ni and Co contents is denoted by x. The atomic ratio of the sum of Ni and Co contents to the sum of Ni, Co, and Fe contents is denoted by y, in other words, the atomic ratio of Fe content to the sum of Ni, Co, and Fe contents is denoted by 1xe2x88x92y. Similarly, the atomic ratio of the sum of Ni, Co and Fe contents to the sum of Ni, Co, Fe, B contents is denoted by z, in other words, the atomic ratio of B content to the whole contents is denoted by 1xe2x88x92z.
According to the invention, the magnetic thin layer contains B, as well as Ni, Co, and Fe that are ferromagnetic metallic elements. B is added in order to improve the thermal resistance, that is, thermal stability of the magneto resistive sensor of the invention, and should satisfy the relationship: 1xe2x88x92z greater than 0. If the content of B is higher than 10 atomic % (at. %), the content of the ferromagnetic metallic elements is insufficient so that the magnetization of the magnetic thin layer is reduced, and therefore, the magneto resistance ratio is decreased. Then, the content of B should be equal to or less than 10 at. %, that is, the relationship: 1xe2x88x92zxe2x89xa60.1 (0.9xe2x89xa6z) should be satisfied.
Among the ferromagnetic metallic elements, Fe is an element with a large ferromagnetic spin. Therefore, the magnetic thin layer has a large spontaneous magnetization by containing Fe. In an artificial lattice GMR multi-layered film, the magnetic thin layers with a nonmagnetic thin layer interposed therebetween have spins alternately oriented in opposite directions and coupled antiferromagnetically, so that the magneto resistive effect can be enhanced by increasing the spontaneous magnetization of the magnetic thin layer. Therefore, the ratio of Fe content to the sum of the other ferromagnetic metallic elements"" contents should be larger than 0. If Fe is not contained, the magneto resistance ratio is reduced and the output is decreased. That is, the inequality: 1xe2x88x92y greater than 0 (1 greater than y) should be satisfied. When the ratio of Fe content is higher than 0.30, the magnetostriction becomes too large in a normal direction so that the hysteresis becomes large, and the magneto resistance ratio becomes 2% or less. Thus, the inequality: 0.70xe2x89xa6y less than 1.00 should be satisfied.
Furthermore, in the case of y less than 0.7, the NiFeCo alloy has a transformation point within the category temperature range. The alloy is a face-centered cubic (fcc) lattice at a temperature higher than the transformation point and is a body-centered cubic (bcc) lattice at a temperature lower than the transformation point. The transformation point may occur at about 200 degrees centigrade, so that when the temperature is higher than 200 degrees centigrade, the magnetic anisotropy and antiferromagnetic coupling may be lost, and thus the GMR functionality may be lost.
As described above, the magnetic thin layer according to the invention essentially contains the elements of Ni and Fe. Substituting part of Ni with Co allows the magneto resistance ratio to be increased. When the Ni content becomes as low as the Co content or lower than it, a saturation magnetic field becomes large so that the sensitivity of the magnetic sensor is degraded. Therefore, the atomic ratio x of Ni content to the sum of Ni and Co contents is required to be higher than 0.50. Thus, the inequality: 0.50 less than xxe2x89xa61.00 should be satisfied. Here, the relationship: x=1.00 refers to the case where Co is not contained.
Although substituting Ni with Co allows the magneto resistance ratio to be improved, the saturation magnetic field is also increased as the substituent Co content is increased. Thus, more preferred ratio of Ni content is 0.60 less than x less than 1.00.
For the nonmagnetic thin layer used in the magneto resistive sensor according to the invention, Cu, Ag, Au, Pt or the like may be used. Among these, Cu is the most suitable in terms of cost.
In order to obtain a high magneto resistance ratio, it is preferable that the magnetic thin layer is 5 to 30 angstrom thick, and the nonmagnetic thin layer is 5 to 30 angstrom thick. In particular, when the magnetic thin layer is 10 to 22 angstrom thick, and the nonmagnetic thin layer is 20 to 25 angstrom thick, a high magneto resistance ratio of 6% or higher can be obtained. If the thickness of the magnetic thin layer is 30 angstrom or more, the interlayer distance between the magnetic elements becomes relatively large, the antiferromagnetic coupling is degraded, and the magneto resistance ratio is decreased. If the thickness of the magnetic thin layer is 5 angstrom or less, the continuity of the magnetic thin film is degraded and the ferromagnetism is not exhibited. If the thickness of the nonmagnetic thin layer is 30 angstrom or more, the distance between the magnetic thin layers is widened, and the antiferromagnetic coupling is degraded. Furthermore, if the thickness of the nonmagnetic thin layer is 5 angstrom or less, the uniformity of the thickness thereof cannot be maintained and ferromagnetic coupling occurs between the magnetic thin layers, so that the magneto resistive effect cannot be obtained.
In the magnetic laminated film used in the invention, the number of the magnetic thin layers alternately laminated with the nonmagnetic thin layers is preferably 5 to 25, more preferably, 10 to 24. Since if the number of laminated layers is small, scattering probability of free electron is low so that a sufficient magneto resistance ratio cannot be obtained, the number of the laminated layers should be 5 or more. On the other hand, if the number of the laminated layers is too large, the thickness of the magnetic laminated film becomes ununiform, and the magneto resistance ratio becomes lower. Each layer may vary in thickness in the plane and have a thickness distribution to some extent. With a small number of laminated layers, even if there is such a thickness distribution, the magneto resistance ratio of the magnetic laminated film is not decreased. However, with an increased number of the laminated layers, their respective thickness distributions are added up, so that the thickness of the entire magnetic laminated film becomes unnuniform, and the magneto resistance ratio is decreased. Thus, the number of the laminated layers should be 25 or less.
A glass substrate can be used as the nonmagnetic substrate. Another nonmagnetic substrate may be used. Preferably, a glass plate having alumina or the like vapor-deposited thereon is used as the nonmagnetic substrate. The magneto resistive sensor according to the invention typically has a structure having the nonmagnetic substrate, and the underlayer, such as a Ta film, deposited on the substrate by sputtering, and the magnetic laminated film provided thereon. However, the magneto resistive sensor according to the invention preferably uses the NiCr alloy or NiFeCr alloy film that is obtained by substituting part of Ni with Fe as the underlayer. Providing this alloy thin film allows the magneto resistance ratio of the magnetic laminated film to be further increased. The alloy thin film can be represented by the formula of composition: (NiaFe1-a)bCr1-b, where 0.4xe2x89xa6axe2x89xa61.0, and 0.4xe2x89xa6bxe2x89xa60.8. If the Cr content in the alloy is lower than 20 at. %, the orientation of the magnetic thin layer deposited thereon is degraded. If the Cr content in the alloy is higher than 60 at. %, the effect of increasing the magneto resistance ratio is lost. Thus, the inequality: 0.6xe2x89xa71xe2x88x92bxe2x89xa70.2 (0.4xe2x89xa6bxe2x89xa60.8) should be satisfied. While substituting part of Ni with Fe allows the magneto resistance ratio to be increased, the atomic ratio of Fe content to the sum of Ni and Fe contents should be 0.6 or less. If the ratio of Fe content is higher than 0.6, the crystal structure of the film is changed so that the film characteristics are changed. Within the range of 0.4xe2x89xa6axe2x89xa61.0, the alloy thin film is the face-centered cubic lattice and is stable. However, the region of a less than 0.4 is a coexistence region of the face-centered cubic lattice and body-centered cubic lattice, where various physical quantities and magnetic characteristics of the NiFeCr alloy exhibit thermal hysteresis. The thermal hysteresis extends to the category temperature range, so that the magnetoresistance ratio of the magnetic laminated film deposited thereon is decreased when the temperature is raised. Thus, the inequality: 0xe2x89xa61xe2x88x92axe2x89xa60.6 (0.4xe2x89xa6axe2x89xa61.0) should be satisfied.
The (NiaFe1-a)bCr1-b underlayer is preferably 10 to 100 angstrom thick. The thickness of 10 angstrom or more allows the magneto resistance ratio to be increased. In order to further increase the magneto resistance ratio and improve the thermal resistance, the thickness is to be 50 angstrom or more. However, if the thickness of the underlayer is larger than 100 angstrom, the uniformity of the thickness of the magnetic laminated film deposited thereon is degraded and the magneto resistance ratio is decreased.
The magneto resistive sensor of the invention described above can have a large magneto resistance ratio of 12 to 18%. As for the resistance of the magnetic laminated film having a direct current applied thereto in a direction along the magnetic laminated film, assuming that the resistance value when the magnetic field is not applied thereto is denoted by R0, the resistance value when the magnetic field is applied thereto is denoted by R, and the resistance variation xcex94R is represented by the formula: xcex94R=Rxe2x88x92R0, then the magneto resistance ratio is denoted by xcex94R/R0.
In addition, the magneto resistive sensor of the invention is less susceptible to decrease in magneto resistance ratio due to a high temperature applied thereto during manufacturing process of the magnetic encoder using the sensor. The magneto resistance ratio is scarcely decreased at 200 degrees centigrade, and is decreased by about 1% at 250 degrees centigrade.
The magnetic encoder for detecting a transposition of a magnetic medium according to the invention includes a magnetic medium having magnetic patterns recorded thereon and a magneto resistive sensor facing the magnetic medium via a magnetic gap and being relatively movable to the medium. The magneto resistive sensor includes a nonmagnetic substrate, an underlayer deposited on the substrate, and a magnetic laminated film formed on the underlayer, in which the magnetic laminated film has a plurality of magnetic thin layers and a plurality of nonmagnetic thin layers alternately laminated. The magnetic thin layer is 5 to 30 angstrom thick, and the nonmagnetic thin layer is 5 to 30 angstrom thick. The magnetic thin layer has a composition represented by the formula: [(NixCo1-x)yFe1-y]zB1-z, where 0.50xe2x89xa6xxe2x89xa61.00, 0.70 xe2x89xa6y less than 1.00, 0.90xe2x89xa6z less than 1.00. Preferably, in the formula of composition, x follows the inequality: 0.60 less than x less than 1.00.
Since the magneto resistance ratio of the magneto resistive sensor used in the magnetic encoder is large, a good sensitivity is provided, and thus the magnetic gap between the magnetic medium and the magneto resistive sensor may be 2 micrometers to 2 mm.